The interfacial tension is a fundamental property of liquid-liquid systems and governs the mixing potential of two immiscible fluids. For two fluids to form a stable mixture, their interfacial tension needs to be very low. Applications of systems with a low interfacial tension find a plethora of applications: for example, in the food industry, for generating oil-water emulsions, or in the pharmaceutical industry, where characterising the mixture is important for the drug delivery. Despite the numerous industrial applications, characterising low interfacial tension remains a challenge. Traditional direct force measurement techniques, such as the Wilhelmy Plate method, have difficulties resolving the small forces involved. Other conventional drop distortion techniques, such as spinning drop tensiometry, require long calibration time and can have error margins up to 60%. This thesis investigates the potential of utilising the optical tweezers for studying low interfacial tensions and developed a method to do so. In this set-up, a highly focused laser beam holds a micron-sized sphere: similar to a pair of tweezers. As light refracts through the bead, momentum is transfered and thus exerts a force. The light refraction is also used for position detection, allowing for a spatial resolution of 0.1-2 nm. As a micro-sphere, which is hold by the optical tweezers, is pushed against an interface, the forces exerted on the bead are probed allowing for the interfacial tension to be determined. Combining the excellent spatial resolution of the optical tweezers with fitting a minimal surface model, the set-up has the potential to measure the interfacial tension over a range of decades. We have researched the dodecane-water interface, where we added glycerol to the aqueous phase to match the index of refraction. The interfacial tension was lowered by adding Span-80 or the combination of Span-80 and SDS. We have been able to conduct several measurements on interfaces with estimated values of O(-6) - O(-3) N/m. We find that the system is highly dependent on the different in index of refraction between the two fluids as well as the curvature of the interface. However, discrepancies between literature values and measurements arise due to the altering of the fluid properties and interfacial dimensions. We conclude that the use of optical tweezers to measure low interfacial tension is promising, however its speed and accuracy are reduced due to the strong requirement to match the index of refraction.

In this thesis a method of combined PIV and LIF experiments on the Turbulent/Non-Turbulent interface (TNTI) of a round jet is presented. By varying the distance from the nozzle in the measurements, the Kolmogorov scale of a large range of Reynolds number (2×103 ≤ Re ≤ 48×103) is kept constant, allowing for a constant resolution. The large range in Reynolds number is needed to identify a difference in scaling of the TNTI, which is difficult to capture due to the weak dependency of the separation of Kolmogorov and Taylor scales on the Reynolds number (λ/η ∼ Re1/4). A factor of over 2.5 is achieved in the current work. An experimental setup for these measurements is presented, as well as the design of experiments for seven Reynolds numbers based on the boundary and initial conditions of the setup. A design for measurements with a camera moving with the flow is presented as well. Conditional statistics are used on the jet to look at the change of span-wise vorticity over the interface. For the detection of the interface scalar images from LIF are used, where the interface is detected using a threshold of dye concentration (Rhodamine B). All Reynolds numbers show self-similarity within the measured range, comparable to literature and with a spatial resolution better than 4η. Some small differences between the low and high Reynolds number measurements can be seen. They are not expected to have significant impact on the results and can likely be solved by slight changes in the experimental setup. The influence of the threshold on the shape and width measurements of the mean conditional vorticity profile is examined, indicating a significant dependency on chosen threshold. Current results indicate a scaling of the interface thickness with the Taylor length scale, around a value of 0.5-0.6λ, or 10-15η This is in accordance to the literature for low Reynolds numbers. Due to this significant influence of the threshold on the outcome, these results cannot be seen conclusive. For this, further investigation is needed on the dependency of the width on the threshold, and validation of the jet contour. Universal ways to compute the threshold, jet contour and mean conditional vorticity profiles are currently missing, while they have shown to be potentially of significance on the outcome. Another method of studying TNTI scaling is via the local velocity scales, which can be found via current data, which could provide a comparison of methods. The measurements with the moving camera could help identifying the link between physical processes and the scaling of the interface.

Turbulent scalar mixing is prominent in all sorts of situations, being it in air pollution or making tea. However, this does not often lead to a homogeneously mixed fluid, but rather leads to regions of higher concentrations next to regions with significantly lower concentrations. This can be characterized by dividing the concentration field in a turbulent flow into a few regions that have almost the same concentration: Uniform Concentration Zones. How these Uniform Concentration Zones are organized is yet unclear, but a possible candidate in the velocity field is the use of Lagrangian Coherent Structures. These structures are, in principle, transfer barriers in the flow and defined as local maxima of the Finite Time Lyapunov Exponent (FTLE), which is a measure of exponential fluid-separation over time. In order to successfully apply the concept of Lagrangian Coherent Structures in an experiment, turbulent structures should remain in focus long enough to measure the exponential separation of fluid parcels. As almost all fully developed turbulence comes with a mean flow, the observation time in an Eulerian field-of-view is too short. Therefore an experiment was designed to move the observation equipment with the mean flow. With this experimental setup, the concentration field and the velocity field are measured simultaneously via respectively LIF and PIV. Comparing the Uniform Concentration Zones with both the forward and backward time FTLE-field, shows that there is not much correlation between the concentration field and the forward time FTLE. However, local maxima of the backward time FTLE seem to overlap often with boundaries between Uniform Concentration Zones.

At the turbulent/non-turbulent interface (TNTI) of a jet flow, momentum is transferred from the turbulent jet fluid to the fluid at rest. This transfer is governed by two mechanisms. One of them is related to the large scales of the flow and the other relates to the small scales of the flow. Which of the two is dominant is still a point of discussion. To analyse the TNTI the instantaneous information of the flow and the location of the interface is of great importance. However, the interface simultaneously develops and travels downstream from the nozzle. With stationary measurement techniques this limits the number of frames the interface can be seen developing as it travels in and out of the FOV. In this thesis the TNTI is measured using a camera system that moves along with the TNTI to get high resolution instantaneous measurements of the same part of the evolving interface. The measurement techniques that are used for this experiment are Particle Image Velocimetry (PIV) and Laser Induced Fluorescence (LIF). The cameras for these measurements are mounted to a motorised frame to keep the same part of the interface in view of the cameras as it develops. Three Reynolds numbers are measured with this setup and the cameras move approximately 50Dn at a velocity close to the velocity of the interface along a diagonal path to follow the evolution of the TNTI. One measurement with a Reynolds number of approximately 1.2 × 104 has been processed to show the quality of the results that can be obtained from such a measurement. The velocities are computed by an in-house interrogation analysis program in MATLAB to overcome the wide range of particle displacement found in this experiment, due to the presence of both the centreline and the TNTI of the jet. The TNTI is detected using the LIF data and a threshold detection method from literature that determines a threshold value. The results from the PIV and LIF processing is combined to compute the average conditional vorticity over the interface. The results show that the PIV analysis is able to compute the velocities of almost the entire jet. Only showing a lot of spurious vectors close to the nozzle in the core of the jet. The LIF edge detection algorithm, on the other hand, does not perform as expected. In multiple instances, an internal interface is detected instead of the TNTI. The TNTI is also determined by identifying a threshold value through a visual inspection of the LIF images. This manually determined TNTI is used as a point of comparison for the average conditional vorticity profiles. The average conditional vorticity profiles support the conclusion that an interface internal to the jet is detected when comparing the interface from the algorithm to the interface determined manually. Although not quantified in this thesis, there are moving measurements that show the same section of the TNTI evolving for many frames. This gives the co-moving measurements a clear advantage in measurement time compared to stationary measurements when trying to measure the moving TNTI. Refinements to the experimental setup, the behaviour of these internal interfaces and the detection of the TNTI can be of interest for future research.

During front crawl swimming, water is driven backwards with the limbs. Drag forces generated by the limbs are consequently used for forward propulsion. The hands are responsible for approximately 60% of the generated propulsive forces. Reaching a podium place in competitive swimming is dependent on differences in finishing times smaller than 0.5%. For this reason, investigating the effects of hand configuration on swimming performance is of interest. Configurable properties of the hand are the finger spreading and hand palm cupping. It is argued that a small finger spreading leads to a larger obstruction in the fluid flow compared to closed fingers, resulting in larger generated drag forces. Similarly, a small hand cupping is expected to increase the drag forces in analogy to the drag increase experienced by cupped disks. In this thesis, an experimental investigation is carried out to look into the effects of both finger spreading and hand cupping. Furthermore, CFD simulations are used for the abstract modelling of hands with finger spreading by use of slotted disks.  Towing tank experiments in water are performed to investigate the effects of finger spreading for five full-scale arm models. The research showed that a small finger spreading of 5° can increase the drag coefficient of the hand with 1.7%, in comparison to closed fingers. Larger spreadings were found to influence the drag coefficient disadvantageously, where a 20° finger spreading reduced the drag with 1.5%. The found effects indicate that finishing times can be reduced with 0.3% by using 5° finger spreading instead of 20° spreading. Wind tunnel experiments are used to look into the effects of hand cupping. Dynamic scaling based on the Reynolds number is used to account for the used air flow. Effects for five full-scale arm models with different hand cuppings were investigated, these have a 5° finger spreading which was found optimal from previous research. It appeared that small rotations around the longitudinal axis of the arm have large influences on the drag coefficient, where a maximum in drag was never found for the hand palm perpendicular to the flow, but with an abducted thumb opposing the flow. The research showed that 6% more drag is generated for a flat hand in comparison to the largest investigated hand cupping. This indicates that finishing times can be reduced with 0.8% by using a flat hand instead of a large hand cupping.  In conclusion, the research found a hand configuration with 5° finger spreading and a flat hand palm optimal for maximizing drag forces. The found effects on finishing times indicate that using this hand configuration can play an important role in reaching podium places during front crawl swimming.

Swimming is a sport where an incremental gain in performance can lead to either a medal or leaving the swimmer empty-handed. In the front crawl, water is driven backwards by the limbs where most propulsive forces come from the hands. Two main stroke patterns can be observed, the ”straight I pull” is commonly taught to generate forces with drag, and the ”curved S-pull” applies lift as well. This investigation uses a new industrial 6-DOF robot arm to study angle-dependent forces, stroke patterns and hand configuration variables. The forearm and hand are analysed using force measurements and PIV to obtain quantitative information, while a parameterisation of the forearm and hand is used to obtain qualitative analysis. The flow is analysed qualitatively using two analytical methods, but it is discovered that these methods can only be applied to a limited range of cases to predict the angle-dependent lift force. Numerical investigations into the parameterised arm model reveal that forces generated were similar in magnitude to those reported in the literature for a pseudo-transient simulation. Quantitative research shows that lift only surpasses drag at extreme angles of attack immediately after a rapid start. This provides clear evidence that drag-based propulsion is preferred for front-crawl swimming. We utilize PIV technology to visualise the flow field around two cases where the lift was highly prevalent. The results reveal various aspects, including the nature of the observed lift peak. Finally, the study compares three distinct strokes: one utilizing a straight drag-based approach and two utilizing a sinusoidal path, resembling a curved pull. The findings indicate that the sinusoidal path’s lift-to-drag ratio remains relatively consistent compared to steady-state, constant-angle experiments. Additionally, the results suggest that the straight stroke is optimal for propulsion, while one of the sinusoidal strokes might be more energy-efficient. An additional aim of this research is to determine the suitability of an industrial 6-DOF robot arm for stroke experiments. It has been discovered that some experiments need a longer path and should preferably be conducted in a wind or water tunnel.

Convective heat transfer finds applications in several domains of industry like heat exchangers, gas turbine blades, IC engine surfaces etc. The surfaces of these heat transfer applications are either naturally rough owing to manufacturing techniques or become rough over a period of time during operation. These rough surfaces usually contain multiple length scales and exhibit random and heterogenous properties. Fractal roughness, in principal, is characterized by self-similar detail on smaller and smaller length scales and hence fractal dimension which is independent of any length scale can become a very viable option to characterize these multi-scale random surfaces. Therefore in the current thesis, rough surfaces characterized by fractal dimensions were designed and their role in the heat transfer enhancement relative to the pressure drop was studied using DNS. The fractal surfaces were designed by randomly placing five generations of self-similar cuboids with decreasing sizes from higher to lower generations. The quantity of cuboids sprinkled randomly for each generation followed a fractal dimension. Two principal fractal dimensions were selected, i.e 퐷 = 1 and 퐷 = 2 and eight random realizations of each were generated in order to study the averaged effect of the heat transfer performance. Since the cuboids were randomly placed, different realizations within the same fractal dimension experienced varied sheltering effects by larger generation cuboids. This ultimately produced a fluctuation in the ”sheltered” solidity of the fractal surface for different realizations whose effect was also studied. The cuboids were resolved in the simulation using an immersed boundary method. To quantify the heat transfer performance of the rough surfaces, two performance factors namely the aero-thermal efficiency and Reynolds analogy factor were defined. It was found that the two types of fractal surfaces performed approximately similarly with a slight higher mean for 퐷 = 2 based on the above heat performance parameters even though the two surfaces looked very different. However, the above performance factors showed a very strong correlation with the ”sheltered” solidity of the fluctuating realizations displaying an increasing trend.